Growth and development in the mammal differ during various periods of life and are influenced by somatotrophin and thyroxine. Somatotrophin regulates skeletal growth by the induction of a secondary plasma mediator, somatomedin, which acts directly on cartilage by stimulating the uptake of sulfate. Thyroxine also influences skeletal growth and acts synergistically with somatotrophin. Since the responsiveness of the tissues is altered during various stages of life the interrelationship of somatotrophin, somatomedin and thyroxine in the fetal, neonatal, immature and adult rat will be investigated. It is well documented that thyroid hormones influence the rate of liver regeneration. Also, from liver regeneration and perfusion studies, it appears that the liver is a principal source of somatomedin. Therefore, we will evaluate whether the influence of thyroxine on liver regeneration will alter the production of somatomedin. A radioimmunoassay of plasma somatotrophin, biological assay of somatomedin and a thyroxine assay will be used. The metabolic status of the animals will be evaluated using basal metabolic rates, liver oxygen consumption and succinic dehydrogenase activity. BIBLIOGRAPHIC REFERENCES: Gaspard, S.T., R. Wondergem and H.M. Klitgaard. Effect of growth hormone and thyroxine on serum somatomedin and growth. Fed. Proc. 34(3):272, No. 307, 1975.